We examined the effects of ST-91 microinjected into the medullary dorsal horn (MDH) on the ability of monkeys to detect small temperature increases in the noxious heat range. Behavioral detection latency and the percentage of correct detections were used as measures of the monkeys perceived intensity of noxious thermal stimulation. The monkey was trained to detect temperature changes of 0.4, 0.6, and 1.40 degrees C superimposed on an initial temperature shift of 46 degrees C (T1). ST-91, an alpha 2 adrenergic agonist, was microinjected (1,3,10 micrograms) into the MDH and produced dose-dependent and stimulus-dependent increases in the detection latency to T2 noxious heat stimuli. There was not effect of ST-91 on the innocuous cooling or light trials indicating that the changes were modality specific and independent of motivation, attention and motoria factors The data demonstrate a pharmacologically-specific effect of an alpha-2 adrenergic agonist on the perceived intensity of noxious heat stimuli at the earliest central relay for noxious information.